The technical field of the present invention is stepper motors, and more particularly, is power maximizing circuits for such motors.
A stepper motor provides controllable speed or position in response to input step pulses commonly applied from an appropriate control circuit. Since the stepper motor increments in a precise amount with each step pulse, it converts digital information, as represented by the input step pulses, to corresponding incremental rotation. By increasing the rate of the step pulses, it is possible to increase the speed of the motor.
Since stepper motors have been in existence, there has been a continuous attempt to obtain more power from the motor by developing higher torque at higher rates or speeds. Because of winding inductance, it becomes more difficult to rapidly conduct current into the windings as the step rate increases. Early stepper motor drive circuits included additional resistance in series with the motor windings so that the inductive time constant was reduced. Another method was to supply a higher voltage during the initial period of each phase change. Later controls introduced the concept of chopping the winding voltage at a two to five kilohertz rate that allowed elimination of the series resistance and improved drive system efficiency. Some systems have been known to use chopped frequencies into the 20 kilohertz range.
Another example of prior approaches to the problem is shown in U.S. Pat. No. 3,967,179 (Loyzim-June 29, 1976). Such prior circuits vary the voltage supplied to the motor windings using two separate circuits, a holding circuit for stand-still or low frequency operation, and a running circuit for higher stepping rates. Each of the circuits runs in synchronism with the step pulses and includes a pulse width modifier and adjustable monostable circuit. Such circuits are digital in nature and are confined to a change of state of the monostable circuit during each step pulse. In addition to this limitation, such circuts can accommodate only a limited number of discrete voltage levels to span the entire range of desired motor speeds. Experience has shown that the duplication of digital circuitry required by the prior circuits substantially increases the cost of the stepper motor as a whole. In addition, the Loyzim circuitry provides no feedback from the voltage suppied to the motor. As a result, the circuit cannot provide correction if the motor voltage is not at the desired level. In addition, such prior circuits provide no means of resonance compensation.